I agree you should probably separate them. Sounds very stressful for little Bella and yes stress can certainly cause runny poops.
Maybe separate cages but some out of cage, supervised play time together?
Hopefully they will eventually get along but there are times when, just like people, 2 birds just plain don't like each other. Or in this case 1 that doesn't like the other
I hope you're able to work it out so they become friends